I keep getting full page advertisements coming up on my computer how do i get rid of them?

After letting my pesky kids on the comp it has suddenly decided i,m a prime candidate for playing poker online or i,m in heavy debt that need to be reconciled with my house as guarantor to say a few of the legitmate companys that seem to want access to my bank account. How the i get rid of the f@ckers. Any help will be gratefully appreciated. Thank you